BSD DevCenter
oreilly.comSafari Books Online.Conferences.

advertisement


Big Scary Daemons

Wireless Networking

04/05/2001

Today I made my wife happy. Yesterday, a 50-foot Ethernet cable ran across the living room floor, under the dining room table, over the gerbil cage, down the cold air return grate, through the basement rafters, and into my basement office hub. Today, it's gone. Instead, a little PC card sticks out two inches or so from the side of the laptop. Here's what you need to know to set up a wireless network in your home or office.

First of all, wireless networks are deceptively similar to Ethernet. Both are broadcast media, and can actually interoperate via a bridge. If you're familiar with Ethernet, you're 80% of the way towards getting your wireless network set up. The other 20% is enough to drive you nuts if you don't know what's going on.

There's three different standards for wireless communication: HomeRF, Bluetooth, and 802.11b.

HomeRF is an older standard. Throughput peaks at 1.6 meg on a good day, with a maximum range of about 150 feet.

Also in Big Scary Daemons:

Running Commercial Linux Software on FreeBSD

Building Detailed Network Reports with Netflow

Visualizing Network Traffic with Netflow and FlowScan

Monitoring Network Traffic with Netflow

Information Security with Colin Percival

Bluetooth has been promised for quite some time. It's fast, secure, and reliable. No hardware is actually available yet. (This might change by the time this article is printed.) Since neither hardware nor BSD drivers are available, we don't really care about this right now.

802.11b is more expensive than HomeRF, has considerably greater range, and supports speeds up to 11meg/second. This is the most popular option, and the one with the best FreeBSD support, so we'll cover it here.

Related Articles:

802.11b Tips, Tricks, and Facts

Performance Test: 802.11b Takes a Lickin' and Keeps on Tickin'

802.11b is an IEEE standard, much like classic 802.3 Ethernet. This means that products from different vendors are supposed to work reliably together. There are still few enough vendors that this is basically true; interoperability testing is fairly straightforward. Lucent, Cisco, Apple, and 3Com are the major vendors, while smaller companies like D-Link are just starting to enter the fray.

When establishing a wireless system, you need to start up front with some basic decisions about how your setup will work. Do you want to build a separate IP network just for wireless communications, or do you want to bridge your wireless systems into an existing Ethernet?

If all the devices on your network are wireless, all you need is a wireless NIC for each machine. Unlike Ethernet, no central hub is required. This is called "ad-hoc" mode.

If you want to integrate wireless into an existing network, you need to invest in a wireless access point to bridge between the two. This is "infrastructure" mode.

You can use ad-hoc mode in combination with an existing Ethernet, but you can't bridge them together. You would need a router with one wireless interface and one Ethernet interface, and each network needs separate blocks of IP addresses. This might make sense if your network is large enough.

Most people are probably interested in infrastructure mode. No company can afford to simultaneously replace all their NICs with wireless ones, let alone discard all their old-fashioned CAT5 network infrastructure.

Besides, a wireless network will never run as fast as a physical network. Ethernet relies on "collision detection." Only one packet can be transmitted over an Ethernet at a time. When two machines transmit Ethernet packets simultaneously, this is a collision. Both machines wait a random number of microseconds, and transmit the packet again.

An 802.11b network card cannot detect collisions. The noise of transmitting a packet drowns out any possibility of receiving an incoming packet. Worse, if a packet is sent to a workstation at the same time that workstation is transmitting the packet, the received packet vanishes.

To work around this problem, when a wireless card receives a packet, it transmits an ACK (acknowledgment) back to the sender. This actually is more reliable than Ethernet, but increases the amount of traffic sent for each packet.

A wireless network also has serious security implications. First of all, this is all done via radio. Your neighbor, the kid on the street, and the Men in Black in the florist truck can pick up your transmissions. 802.11b encrypts packets via WEP (Wired Equivalent Privacy) protocol. Enough encryption should prevent eavesdropping. WEP is flawed, however; you shouldn't rely upon it.

I work around this on my home network by having every host speak SKIP (/usr/ports/security/skip) when speaking to an internal host. This isn't appropriate for corporate environments, or for most users without too much time on their hands.

If your company decides to roll out wireless networking, you need to be very aware of personnel changes. The HR department needs to notify your IT group of departures immediately, so they can change network names and encryption keys within minutes.

After all, once upon a time a person being fired could be escorted beyond the computer security perimeter. If you have 802.11b, however, that person can sit in the parking lot and access your company network! Many companies with a tight external security policy are more relaxed on the inside. While many Unix admins wouldn't dream of using rlogin on the open Internet, it's second nature inside the building. Wireless requires you to change that attitude.

802.11b transmits at 2.4 GHz, the same spectrum as microwave ovens. The cards use less power than a mobile phone. Cisco warns that their PCMCIA card should be more than 2 inches from your body, and the access point's antenna should be at least 6 inches away from anyone. I hold my laptop on my lap. (That is where the name came from, after all.) If I get cancer on top of my right thigh, I know who to blame.

Now that you're somewhat familiar with the problems, technologies, and risks of wireless networking, we'll get more detailed. The next article will focus on how exactly to set up 802.11b on FreeBSD.

Michael W. Lucas


Read more Big Scary Daemons columns.

Return to the BSD DevCenter.





Sponsored by: